Amplifier employing interleaved signals for PWM ripple supression

ABSTRACT

An amplifier having improved distortion characteristics is set forth. The amplifier includes an interleaved PWM amplifier that generates interleaved PWM pulses in response to a modified input signal and one or more carrier signals. The interleaved PWM pulses of the amplifier are used to drive a power stage, such as an opposed current power stage. The amplifier also includes an interleaved PWM generator that provides interleaved PWM pulses in response to the modified input signal and one or more further carrier signals. The carrier signals used by the PWM generator may differ in phase from the carrier signals used by the interleaved PWM amplifier to generate its interleaved PWM pulses. One or more feedback circuits are employed in the generation of the modified input signal. More particularly, the feedback circuit(s) generates the modified input signal based on an input signal that is to be amplified and the interleaved PWM pulses of the interleaved PWM generator.

This application is a divisional of U.S. patent application Ser. No.11/485,612 entitled AMPLIFIER EMPLOYING INTERLEAVED SIGNALS FOR PWMRIPPLE SUPPRESSION, filed on Jul. 12, 2006, which is incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention generally relates to amplifiers and, more particularly, toan interleaved amplifier employing interleaved signals for PWM ripplesuppression.

2. Related Art

Pulse width modulation (PWM) amplification for audio applications hasbeen used to increase efficiency by incorporating output devices thatact as switches as opposed to linear devices that must dissipate asubstantial amount of power. In PWM amplifiers, an audio input signal isconverted to a pulse width modulated waveform. To this end, an audiosignal is provided to the amplifier to modulate the width of anultrasonic rectangular waveform based, for example, on the amplitude ofthe audio signal. The modulated waveform is used to drive one or moreoutput devices as switches that are either fully saturated or off. Theoutput devices, often implemented using switching power transistors, maybe aligned in half-bridge pairs such that one device of the pairswitches a positive voltage to the output, while the other deviceswitches a negative voltage to the output. The switched output signalsmay be provided to the input of a low-pass filter in an attempt toremove harmonic signals and sidebands that are beyond the spectrum ofthe desired output waveform. The filtered analog signal is used to drivethe load, such as a loudspeaker.

One set of pulse width modulated amplifier architectures, known asclass-D amplifiers, are theoretically 100% efficient because the outputtransistors are either completely on, or completely off. Theseamplifiers, however, may be problematic since the timing of theswitching of the transistors must be precisely controlled. In a class-Damplifier, the switches operate in time alternation. Ideally, theswitching is perfectly timed so that one transistor instantaneouslyturns off as the other instantaneously turns on. If the switching is notperfectly timed, both the positive and negative switching devices may beon at the same time, allowing high “shoot-through” current, which maydestroy the circuitry of a subsequent stage in the amplifier system.Therefore, in practice, a delay may be purposely introduced between thetime at which one transistor turns off and the other transistor turnson. The time between the conduction intervals of the two switches whenneither switch is on is known as deadtime. Deadtime may result indistortion and, therefore, should be minimized. Conversely, aninsufficient amount of that time may result in undesired shoot-throughcurrent.

An amplifier addressing the shoot-through current and deadtime issues isavailable from Crown Audio International of Elkhart, Ind. The amplifierarchitectures used in certain of the Crown Audio amplifiers are known byvarious names including opposed current amplifiers, a balanced currentamplifiers (BCA®), and “I-class” amplifiers. In this amplifierarchitecture, the positive and negative switching pulses correspondingto the modulated waveform are time interleaved with one another. Whenthe audio input signal is at a zero-crossing, i.e. where no signal is tobe provided at the amplifier output, the interleaved pulses turn theswitches on and off in an overlapping manner at a 50% duty cycle. As aresult the positive and negative power sources through-connected by theswitches cancel each other out to provide a null output signal. When theincoming signal that is to be amplified exceeds the zero crossing andenters a positive voltage state, the duty cycle of the interleavedpulses are such that the duty cycle of the switch through-connecting thepositive power source increases. When the incoming signal falls belowthe zero-crossing and goes to a negative state, the converse occurs.

Although the opposed current amplifier architecture provides asignificant improvement over conventional PWM amplifiers, thearchitecture may be the subject of improvements. For example, as will beset forth in further detail below, the distortion results of anamplification system using multiple opposed current amplifiers that areinterleaved with one another may be improved through the use ofintelligently designed feedback systems.

SUMMARY

An amplifier having improved distortion characteristics is set forth.The amplifier includes an interleaved PWM amplifier that generatesinterleaved PWM pulses in response to a modified input signal and one ormore carrier signals. The interleaved PWM pulses of the amplifier areused to drive a power stage, such as an opposed current power stage. Theamplifier also includes an interleaved PWM generator that providesinterleaved PWM pulses in response to the modified input signal and oneor more further carrier signals. The carrier signals used by the PWMgenerator may differ in phase from the carrier signals used by theinterleaved PWM amplifier to generate its interleaved PWM pulses. One ormore feedback circuits are employed in the generation of the modifiedinput signal. More particularly, the feedback circuit(s) generates themodified input signal based on an input signal that is to be amplifiedand the interleaved PWM pulses of the interleaved PWM generator.

Multiple feedback circuits may be employed. To this end, a firstfeedback circuit may be implemented to feedback an output of the powerstage of the interleaved amplifier to generate a first feedback signal,while a second feedback circuit may be implemented to feedback theinterleaved PWM pulses of the PWM generator to generate a secondfeedback signal. A combiner circuit may be used to combine the inputsignal, the first feedback signal, and the second feedback signal togenerate the modified input signal.

The signal transfer characteristics of various amplifier sections may bemanipulated to meet the desired degree of distortion reduction. Forexample, the interleaved PWM amplifier and the first feedback circuitmay combine to exhibit a first signal transfer characteristic, while theinterleaved PWM generator and the second feedback circuit may combine toexhibit a second signal transfer characteristic. The first and secondtransfer characteristics may be selected so that they are proportionalto one another in about the same ratio as N_(L) to N_(N) over at least apredetermined portion of an output bandwidth of the amplifier, whereN_(L) is the interleave order of the interleaved PWM generator and N_(N)is the interleave order of the interleaved PWM amplifier. In someamplifier implementations, the interleaved PWM amplifier may have aninterleave order of one (non-interleaved).

Other systems, methods, features and advantages of the invention willbe, or will become, apparent to one with skill in the art uponexamination of the following figures and detailed description. It isintended that all such additional systems, methods, features andadvantages be included within this description, be within the scope ofthe invention, and be protected by the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the followingdrawings and description. The components in the figures are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention. Moreover, in the figures, likereferenced numerals designate corresponding parts throughout thedifferent views.

FIG. 1 is a schematic block diagram of one example of an interleavedamplifier that may be used to implement the amplifier systems shown inFIGS. 3 through 5.

FIG. 2 is a signal diagram illustrating the operation of the interleavedamplifier shown in FIG. 1.

FIG. 3 is a block diagram of one example of an amplifier employing aninterleaved PWM feedback signal that is generated from a carrier thatdiffers in phase from the carrier used to generate the PWM drive signalsprovided to the power stage of the amplifier.

FIG. 4 is a block diagram of a further example of an amplifier systememploying an interleaved PWM feedback signal that is generated from acarrier that differs in phase from the carrier used to generate the PWMdrive signals provided to the power stage of the amplifier.

FIG. 5 is a block diagram of a still further example of an amplifiersystem employing an interleaved PWM feedback signal that is generatedfrom a carrier that differs in phase from the carrier used to generatethe PWM drive signals provided to the power stage of the amplifier.

FIG. 6 is a block diagram of a still further example of an amplifiersystem employing an interleaved PWM feedback signal, where the desiredcharacteristics of the feedback signal may be determined by thecharacteristics of an attenuator circuit and a delay circuit.

FIG. 7 illustrates a number of interrelated operations that may be usedto implement one or more of the amplifier systems shown in FIGS. 3through 6.

FIG. 8 illustrates a number of interrelated operations that may be usedto implement one or more of the amplifier systems shown in FIGS. 3through 6, where the output signal of the amplifier power stage is usedin addition to the interleaved PWM feedback signal to generate themodified input signal to the interleaved PWM amplifier.

FIG. 9 is a block diagram of a further amplifier system that employsinterleaved PWM signals for PWM ripple suppression.

FIG. 10 illustrates a number of interrelated operations that may be usedto implement the amplifier system shown in FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to comprehend the exemplary interleaved amplifier system setforth below, an understanding of the output signal spectrum of a moreconventional PWM amplifier may be useful. More particularly, the outputsignal spectrum of a natural double-sided PWM process may be representedas:

$\begin{matrix}{\overset{\_}{{y_{0}(t)} = {{{MV}_{0}{\cos( {\omega_{s}t} )}} + {( \frac{4V_{0}}{\pi} ){\sum\limits_{m = 1}^{\infty}\frac{1}{m}}}}}{\quad\begin{bmatrix}{\sum\limits_{n = {- \infty}}^{\infty}{J_{n}( \frac{{Mm}\;\pi}{2} )}} \\{\sin( \frac{( {m + n} )\pi}{2} )} \\{\cos( {{m\;\omega_{c}t} + {n\;\omega_{s}t}} )}\end{bmatrix}}} & ( {{Equation}\mspace{14mu} 1} )\end{matrix}$

-   M is the modulation index where 0≦M≦1.0-   ω_(s) is the signal frequency in radians/second-   ω_(c) is the PWM carrier/switching frequency in radians/second-   V_(o) is the peak output voltage of the PWM waveform-   m is the integer harmonic order number of the carrier band 1≦m≦∞-   n is the sideband order number −∞≦n≦∞

The leading cosine term is the intended signal for a process whose inputis a cosine of radian frequency ω_(s) and whose amplitude relative tothe modulating triangular waveform is of proportion M. The trianglewaveform is given in unity amplitude cosine form as:

$\begin{matrix}\overset{\_}{\overset{\_}{v(t)} = {\frac{8}{\pi^{2}}{\sum\limits_{{m = 1},3,{5\mspace{11mu}\ldots}}^{\infty}{\frac{1}{m^{2}}{\cos( {m\;\omega_{c}t} )}}}}} & ( {{Equation}\mspace{14mu} 2} )\end{matrix}$

The second part of (Equation 1) is composed of harmonics (m) of themodulating triangle and sideband pairs (±n) about each harmonic. Theamplitude of each sideband is given by a Bessel function J_(n) of thefirst kind (order n), a function of the modulation index M, and harmonicorder m. Since m and n are always integers, the product sine term, hasthree possible values, −1, 0 and +1. When m+n is even, the sine term iszero and there is no sideband. Similarly, there is ideally no carrierwhen n=0. In other words, even harmonics have only sidebands that arespaced by odd multiples of the signal frequency, and odd harmonics haveonly even multiples of the signal frequency including a carrierharmonic.

In a conventional PWM amplifier, the feedback of a sideband whose n=1produces a gain error and not a distortion. Terms where n=0 and where mis odd create DC offset errors. These signals result in harmonicdistortion when they produce intermodulation signals that lie within thesignal passband of the amplifier. For an audio amplifier, that signalpassband may be less than or equal to about 20 KHz, but may often be ashigh as 40 KHz in high end audio systems. Signals that lie outside thefrequency passband may be measured and used as an indicator of amplifierperformance.

One manner of reducing undesired signals in the frequency spectrum of aPWM amplifier output signal is to use a power stage that is switchedusing interleaved PWM pulses. When implemented using naturaldouble-sided PWM, such interleaving may result in considerablesuppression of undesired signals over an extensive range of harmonicorders. Further, such interleaving may result in considerablesuppression of sidebands for bands that are not modulo the interleavenumber N of the amplifier.

An exemplary opposed current amplifier that may be used to implement aninterleaved amplifier system is shown generally at 100 of FIG. 1. Theillustrated system includes a pulse width modulator section 105 and anopposed current drive section 110. The pulse width modulator section 105includes an input section 113 including an error amplifier 115. Erroramplifier 115 receives the audio signal that is to be amplified Vin aswell as a feedback signal Vfb. The feedback signal Vfb is subtractedfrom the audio signal Vin by the error amplifier 115 to generate amodified input signal +Vmod at its output. This output is provided tothe input of an inverter circuit 120, which generates an output signal−Vmod that is approximately 180 degrees out of phase with +Vmod.

Signals +Vmod and −Vmod are provided to the input of a positive pulsemodulator circuit 130 and negative pulse modulator circuit 125,respectively. Both modulator circuits 130 and 125 modulate theirrespective input signals +Vmod and −Vmod with a carrier signal that isprovided at one or more lines 135 by a carrier generator 140. When thesystem 100 is used for audio amplification, the carrier signal may beimplemented as an ultrasonic frequency triangle wave. It also may bepossible to use other carrier signal types, depending on the particularapplication.

The output of positive pulse modulator 130 is provided as an input tothe opposed current drive section 110 to control the state of switchdevice 160. As shown, switch device 160 is used to through-connect apositive voltage +Vcc to a load through a low-pass filter when in theconductive state. The low-pass filter in this example is comprised ofinductor 165 and capacitor 155. Similarly, the output of negative pulsemodulator 125 is provided as an input to the opposed current drivesection 110 to control the state of switch device 145. As shown, switchdevice 145 is used to through-connect a negative voltage −Vcc to theload through a further low-pass filter when in the conductive state. Thefurther low-pass filter in this example is comprised of inductor 150 andcapacitor 155. Inductors 150 and 165 may both have the same inductancevalue and characteristics. Switch devices 145 and 160 may beimplemented, for example, using one or more of a variety of differentswitching transistor technologies. Diodes 170 and 175 function asfreewheel diodes during the switching operations.

FIG. 2 illustrates the relationship between different waveforms that aregenerated in the opposed current amplifier shown in FIG. 1. In thisexample, a single triangular carrier waveform 205 (Vcar) is showncentered at 0V, and the modified input signal +Vmod is shown as a dottedline at 210. The pulse signals provided to the switches 160, 145 areillustrated by waveforms 215 and 220, respectively. Pulse signals 215and 220 are provided to the drive circuit in a manner that generates aswitched power voltage, shown here at 225 (Vcombined), that, forexample, may be filtered, processed, or the like, for provision to aload.

For illustration purposes, the modified input voltage 210 is shown at arelative level of 0V from periods t0 through t3 of FIG. 2. During thistime, signals 215 and 220 have the same amplitude and duration. As aresult, the switched power voltage 225 remains at 0V. However, themodified input signal 205 transitions to a voltage below 0V betweentimes t3 and t4, where it remains until time t5. This change in thevoltage level of the modified input signal 205 causes a correspondingchange in the duty cycles of the pulse signals 215 and 220 that, inturn, results in the generation of the negative going pulses illustratedin waveform 225. Similarly, the modified input signal 205 transitions toa voltage above 0V between times t5 and t6, where it remains throughtime t7. This change in the voltage level of the modified input signal205 likewise causes a corresponding change in the duty cycles of thepulse signals 215 and 220. In this latter instance, the duty cyclechange results in the generation of the positive going pulsesillustrated in waveform 225.

The output spectrum of such an interleaved PWM amplifier is given by:

$\begin{matrix}{\overset{\_}{{y_{0}(t)} = {{{MV}_{0}{\cos( {\omega_{s}t} )}} + {( \frac{4V_{0}}{\pi} ){\sum\limits_{m = 1}^{\infty}\frac{1}{pN}}}}}{\quad\lbrack {\sum\limits_{n = {- \infty}}^{\infty}{{J_{n}( \frac{{MpN}\;\pi}{2} )}{\sin( \frac{( {{pN} + n} )\pi}{2} )}{\cos( {{{pN}\;\omega_{c}t} + {n\;\omega_{s}t}} )}}} \rbrack}} & ( {{Equation}\mspace{14mu} 3} )\end{matrix}$where:

-   N is the integer interleave order 1≦N≦∞; and-   pN is the integer harmonic number and 1≦p≦∞.

When N=1 the above expression is identical to (Equation 1) as expected.Wherever m had appeared in (Equation 1), now pN appears and reveals thatthe interleave architecture has ideally suppressed all bands of order mthat were not modulo N. Those bands (m) that remain as integer (p)multiples of N ideally have identical sideband and carrier harmonics asin (Equation 1).

The foregoing interleaved amplifier analysis reveals that interleavedPWM amplifiers have self-filtering characteristics that render theamplifier capable of selectively removing entire bands of PWM spectrumfrom the signal path. Increasing the order of interleave N of theamplifier results in a corresponding decrease in the amplitude andincrease in the frequency of the PWM spectral remnants in the switchedpower output signal provided from the switched power stage. These lowamplitude, high frequency spectral remnants may be easily filtered fromthe switched power signal to generate an intended output signal to theload, where the intended output signal constitutes a faithfullyamplified version of the amplifier input signal with limited distortion.Further reductions in the corruption of the intended output signalcaused by PWM spectral remnants can be achieved using interleaved PWMpulses to enhance the filtering of low power signals that are corruptedwith the PWM spectrum. This manner of enhancing the filtering of thelow-power signals may be combined, for example, with one or morefeedback methods.

One example of an amplifier 300 that employs interleaved PWM pulses toenhance the filtering of low power feedback signals that otherwise maybe corrupted with the spectrum of the modulating PWM carrier signal(s)is shown in FIG. 3. In this example, amplifier 300 includes aninterleaved PWM amplifier 305 that, in turn, may include pulse widthmodulator 310, output driver 315, and output filter 320. Pulse widthmodulator 310 generates interleaved PWM pulses to the output driver 315based on a modified input signal 325 and one or more carrier signals 330provided by carrier generator 335. The interleaved PWM pulses may begenerated in the manner shown in FIG. 1, which may be expanded to aninterleave order N_(N) based, at least in part, on the number of carriersignals 330 that are modulated by the modified input signal 325. Carriersignals 330 may be in the form of multiple triangular waveforms havingdifferent phases but the same amplitude. The phases of the carriersignals 330 may be chosen so that their corresponding signal vectorsequally divide a unit circle.

Output driver 315 may include one or more switching power stages of thetype shown in FIG. 1. As in FIG. 1, the interleaved PWM pulses providedby the PWM modulator are used to turn the power switching devices, suchas switching transistors and the like, on and off. The resultingswitched power output from output driver 315 may be provided to theinput of output filter 320. Output filter 320 may include one or morefilters to remove signals beyond the desired passband of the amplifier300 before the intended signal 340 is provided to load 345. Anycomponents that may be needed to combine multiple switched power signalsof output driver 315 with one another for provision to load 345 also maybe included in output filter 320.

Although the interleaved architecture of interleaved PWM amplifier 305endows amplifier 300 with self-filtering characteristics that areinherent to the interleaved architecture, further measures are alsoemployed in amplifier 300 to reduce unwanted harmonics, sidebands, andthe like in the intended output signal 340. For example, negativefeedback of the intended output signal 340 and/or the signal 343 fromthe output of the PWM output driver 315 may be used to correctnon-linear and/or stochastic imperfections present in less than idealrealizations of the amplifier 300. Such imperfections naturally occurwhen the amplifier 300 is implemented with actual components, and may becontrasted with the idealized representations of those components oftenused in theoretical circuit analyses.

In the illustrated example, negative feedback is accomplished bydirecting the intended output signal 340 and/or the signal 343 from theoutput of the PWM output driver 315 through a feedback signal path 350to generate one or more negative feedback signals 353 at the input of acombiner circuit 355. The intended output signal 340 and/or the signal343 from the output of the PWM output driver 315 may be processed bycomponents in the feedback signal path 350 to generate feedback signal353 in one or more of a variety of different manners. For example, theintended output signal 340 and/or the signal 343 from the output of thePWM output driver 315 may be filtered, time delayed, phase delayed,scaled, or the like by the components of path 350. The combiner circuit355 shown in FIG. 3 subtracts feedback signal 353 from an input signal360 to complete pursuant to the negative feedback operation. The inputsignal 360 may constitute a signal that is directly provided toamplifier 300 for amplification or, alternatively, may constitute aprocessed signal that corresponds to the signal directly provided toamplifier 300 for amplification.

As noted above, amplifier 300 also employs interleaved PWM pulses toenhance the filtering of low power signals that otherwise may becorrupted with the spectrum of the modulating PWM carrier signal(s) usedby pulse width modulator 310. In the example shown in FIG. 3, aninterleaved PWM generator 370 provides interleaved PWM pulses 380 to theinput of another feedback signal path 375. Feedback signal path 375, inturn, provides one or more further feedback signals 385 to the input ofcombiner circuit 355, where the signal(s) is subtracted from the inputsignal 360 and negative feedback signal 353 to generate the modifiedinput signal 325. The interleaved pulses 380 may be processed bycomponents in the feedback signal path 375 to generate feedback signal385 in one or more of a variety of different manners. For example, theinterleaved pulses may be filtered, time delayed, phase delayed, scaled,or the like by the components of path 375.

Pulse width modulator 370 generates interleaved PWM pulses 380 tofeedback signal path 375 based on the modified input signal 325 and oneor more carrier signals 390 provided by carrier generator 335. Theinterleaved PWM pulses may be generated in the manner shown in FIG. 1,which may be expanded to an interleave order N_(L) based, at least inpart, on the number of carrier signals 390 that are modulated by themodified input signal 325. Carrier signals 390 may be in the form ofmultiple triangular waveforms having different phases but the sameamplitude. Further, carrier signals 390 may differ in phase from carriersignals 330. The phases of the carrier signals 330 and 390 may be chosenso that their corresponding signal vectors equally divide a unit circle.

FIG. 4 is a signal flow diagram illustrating exemplary transfercharacteristics of amplifier 300. In this example, amplifier system 400receives an input signal Ein and generates an amplified output signalEout. In the case of a power amplifier, the output signal Eout woulddeliver energy to a load, such as a loudspeaker. There also may beadditional filtering between the output signal Eout and the ultimateload.

Block 405 represents an interleaved PWM amplifier having a signaltransfer characteristic G_(N). The interleaved PWM amplifier representedby block 405 may have some non-linear and/or stochastic imperfectionsthat are corrected through negative feedback. To this end, the outputsignal Eout is processed through a feedback block 410, which has asignal transfer characteristic H_(N). The output of the feedback block410 is provided to the input of a signal combiner 415, where it issubtracted from the input signal Ein as part of the process used togenerate a modified input signal 420 at the output of the signalcombiner 415. As shown, block 405 uses the modified input signal 420 togenerate output signal Eout.

Block 425 encompasses pulse width modulator 370 of FIG. 3 and has asignal transfer characteristic G_(L). In this example, block 425 may bea linear, low-noise gain block having its output processed by block 430,where block 430 has a signal transfer characteristic H_(L). The outputof block 430, in turn, is subtracted from the input signal Ein bycombiner circuit 415 as part of the process used to generate themodified input signal 420.

The signal transfer characteristic H_(L) of block 430 may be selected sothat G_(L)·H_(L) is nominally proportional to G_(N)·H_(N) over at leasta predetermined portion of the output bandwidth of the amplifier 400. Inmany instances, this proportionality may be generally maintained overthe entire bandwidth for which the amplifier is designed. In choosingsignal transfer characteristic H_(L), the interleave orders of blocks405 and 425 may be considered. Block 405 is understood to have aninterleave order of N_(N), and block 425 is understood to have aninterleave order of N_(L). Consequently, amplifier 400 has a feedbacksystem interleave order of N_(N)+N_(L)=N_(S). Sideband cancellationproperties for an amplifier having a system interleave order of N_(S)are enhanced when the proportionality of G_(N)·H_(N) to G_(L)·H_(L) isapproximately equal to the proportionality of N_(N) to N_(L). Signaltransfer characteristic H_(L) therefore may be selected in accordancewith this attribute.

Realizing the desired proportionality between G_(N)·H_(N) andG_(L)·H_(L) also may entail adding a delay to the output of block 425 orblock 430. This added delay may be used to compensate for the inherentdelay associated with driving the switches in the power stage(s) ofblock 405. Alternatively, or in addition, delay compensation may beincluded in one or both of blocks 425 and 430.

It is possible that N_(N)=1. In such instances, block 405 is notinterleaved and the only interleave order provided in the system is byblock 425, where N_(L)>0. With reference to amplifier 300 shown in FIG.3, such a system may be constructed merely by replacing interleaved PWMamplifier 305 with a standard, non-interleaved PWM amplifier.Notwithstanding this substitution, sideband and harmonic signalreductions are realized when feedback of the interleaved pulses 380through signal path 375 is employed.

From a feedback interleave perspective, it does not matter which phasingvectors represent the carrier signals modulated in blocks 405 and 425 aslong as the Ns vectors evenly divide the unit circle and have amplitudesthat are substantially equal. In one implementation, the phasing vectorsused in block 405 evenly divide the unit circle, and the phasing vectorsof block 425 evenly divide the angles between the phasing vectors ofblock 405. Using such a phasing vector configuration assists inminimizing noise induced errors in the modulators driving the powerstages of block 405.

Each of the blocks 405 and 425 may be elaborated by expanding theirlevel of detail to show the interleaved structure present in each. Insome instances, the interleaved structure in one or both blocks 405 and425 may be implemented through the use of parallel systems. However, theinterleaved structure(s) also may be implemented from series interleavedelements. For example, a classic full-bridge power converter may beimplemented using two half-bridge interleaved amplifiers having theiroutput circuits in series with the load.

An analysis of the amplifier 400 with its corresponding signal transfercharacteristics results in the following closed-loop gain equation:

$\begin{matrix}\overset{\_}{\frac{\overset{\_}{E\;{out}}}{E\;{in}} = \frac{G_{N}}{( {1 + {G_{L}H_{L}} + {G_{N}H_{N}}} )}} & ( {{Equation}\mspace{14mu} 4} )\end{matrix}$

In this equation, the feedback factor is expressed in the denominatorand is dominated by the G·H terms. Since these terms are related to thevalues of N_(N) and N_(L), the feedback factor is diluted as the valueof N_(L)/(N_(L)+N_(N)) increases. Consequently, the values of N_(L) andN_(N) should be considered when assessing the stability of the feedbacksystem. System stability is best when the value of N_(L) is notsignificantly larger than N_(N). For example, the value of N_(L) may beselected so that it is twice the value of N_(N). However, it will berecognized that other relationships between the N_(L) and N_(N) valuesmay be employed while still maintaining system stability.

The matching/ratioing of G_(L)·H_(L) to G_(N)·H_(N) can be approachedfrom the standpoint of sharing as much of signal transfer characteristicH_(L) as possible with H_(N) of the negative feedback signal. Thisapproach may be used to reduce the cost and complexity of the feedbackarchitecture shown in FIG. 4. FIG. 5 shows such an approach in a systemshown generally at 500. As shown, the output of block 405 is provided tothe input of a signal attenuator 505 having a signal transfercharacteristic a, and the output of block 425 is provided to the inputof a delay circuit 510 having a delay value of Δt. The signal attenuator505 may be used to compensate for differences in output gain, since theoutput and gain of signal transfer function G_(N) will be generallylarger than would be expected of signal transfer function G_(L). Thedelay circuit 510 provides time compensation between the output signalsof blocks 405 and 425 since the power stages employed in block 405 tendto have appreciable propagation delays that may require compensation ifthe two signal paths are to be matched in the proper ratio. Anattenuated signal 515 and delayed signal 520 are added to one another bycombiner circuit 525 to generate a feedback signal 530. Block 535 has asignal transfer characteristic H and represents optional and/orunintentional signal processing/distortions to which the feedback signal530 may be subjected. The output of block 535 is a primary feedbacksignal 540 that is combined with input signal Ein by combiner circuit545 to generate the modified input signal 420. In the system 500, thedesired N_(N) to N_(L) ratio can be achieved by adjusting the parameterswithin α, G_(N) and G_(L).

The system 500 shown in FIG. 5 may be employed in a multichannelamplifier where the modulation waveforms for higher order interleave mayalready exist. For example, in a two channel amplifier that already hasan interleave of two in each channel, the second channel may have amodulating waveform that is formed in time quadrature with themodulating waveform used by first channel to result in a sum-of-channelsoutput bridging that has an interleave of four. In this situation, thePWM modulator for block 425 need only employ two comparators and may beprovided with the same main signal inputs as the comparators used in themodulator(s) of block 405. Once delay has been added to the comparatoroutputs of block 425, these output signals are already in a state inwhich they are ready to be combined with the attenuated version of thesignal from the main output.

Depending on design criterion, the signal transfer function G_(L) may betreated as entirely linear except for an idealized PWM modulationprocess. However, it also may be desirable to intentionally add somenon-linearity to G_(L) to help correct for non-linearities of signaltransfer function G_(N). When the form of error is identical for bothsignal transfer functions, then there is no dilution of the effectivefeedback factor that results from this technique as regards distortioncorrection. If the distortion in G_(L) is overstated, then it ispossible for some localized distortion nulling to occur in the output.In such a system, the distortion exhibited by signal transfercharacteristic G_(L) may naturally resemble some of the distortionexhibited by signal transfer characteristic G_(N) since the mechanismsthat create propagation delays also introduce some of the distortionsthat are common to both. Therefore, if either or both block 425 and/ordelay circuit 510 are designed to include some of the same propagationdistortion characteristics as generated in block 405, like distortionswill be found on signals 515 and 520. Ultimately, these distortions willbe used to generate modified input signal 420 and, in turn, result in areduction of the effects of propagation delay distortions on the outputsignal Eout.

The foregoing systems may be implemented in the analog domain, thedigital domain, or a combination of both. FIG. 6 illustrates one mannerin which such systems may be implemented in a combined domain. As shown,amplifier system 600 includes a modulator section 603 and a switchedpower section 605. Switched power section 605 includes an opposedcurrent switching circuit 607 that is responsive to PWM pulse drivesignals 610 provided from the modulator section 603. The output of theopposed current switching circuit 607 may be provided to a summingcircuit/output filter 613 that, in turn, provides the intended outputsignal 615 to a load 617. The intended output signal 615 may be used fornegative feedback by providing it to the input of a combiner circuit 620of the modulator section 603. Alternatively, or in addition, the drivesignals provided at the output of the opposed current switches may beprovided to the input of the combiner circuit 620 along lines 619 foruse in providing negative feedback. The combiner circuit 620 alsoaccepts the signals at one or more lines 657. The output of the combinercircuit 620 is provided at one or more lines 623, which, in turn, isprovided to the input of an analog-to-digital converter 625. Prior toits provision to the analog-to-digital converter 625, the signal(s) 623may be scaled, filtered, or otherwise processed in the analog domain sothat further feedback processing in the digital domain may besimplified, if desired.

The analog-to-digital converter 625 provides output signals at one ormore lines 627 that correspond to the signals 623. These output signalsare digitally processed by a feedback processor 650 along with digitalsignals 630 to generate a modified input signal 633. Digital signal 630corresponds to an analog signal input 635 that has been converted to adigital format by analog-to-digital converter 637. Input signal 635 mayconstitute a signal that is directly provided to amplifier 600 foramplification or, alternatively, may constitute a processed signal thatcorresponds to the signal directly provided to amplifier 600 foramplification.

Modulator section 603 includes a multiphase carrier generator 640 thatgenerates digital representations of the various carrier signals thatare PWM modulated using the modified input signal 633. In theillustrated system, a digital representation 643 of a first set of oneor more analog carrier signals is provided to the input of interceptpredictor 645. A digital representation 647 of a second set of one ormore analog carrier signals is provided to the input of interceptpredictor 651. The digitized carrier signals 643 and 647 may correspondto triangular modulator signals. The analog carrier signals representedby digitized carrier signals 643 differ in phase from the analog carriersignals represented by digitized carrier signals 647. In the illustratedimplementation, the analog carrier signals represented by digitizedcarrier signals 643 may have phase vectors that evenly divide a unitcircle. Similarly, the analog carrier signals represented by digitizedcarrier signals 647 may be selected so that they evenly divide a unitcircle at angles between the phase vectors corresponding to the analogsignals represented by digitized carrier signals 643.

The modified input signal 633 may be compared with the digitized carriersignals 643 by the intercept predictor 645 to determine where signals633 and digitized carrier signals 643 will intercept one another. It ispossible to use multiple input data samples on both sides in time of theexpected time intercept point to compute a point of intercept. Thedetermination of the intercept points can be calculated usinginterpolation/root-finding software implemented in, for example, adigital signal processor. Likewise, the modified input signal 633 iscompared with the digitized carrier signals 647 by the interceptpredictor 651 to determine where signals 633 and digitized carriersignals 647 will intercept one another. Data indicating an interceptbetween signals 633 and 643 is provided to pulse generator 653 togenerate interleaved PWM pulses 610.

Data indicating an intercept between signals 633 and 647 may be used ina number of different manners to achieve the desired feedback effects.For example, the intercept data may be provided to the input of a pulsegenerator 655 to generate interleaved PWM pulses 657 to the input of thecombiner circuit 620. Alternatively, the digital output of interceptpredictor 651 may be fed directly to the feedback processor 650. Stillfurther, intermediate processing of the output of the interceptpredictor 651, pulse generator 655, and/or combiner circuit 620 may beemployed to, for example, perform any N_(N) to N_(L) ratio matching thatmay be desired.

Many of the components of amplifier 600 may be integrated with oneanother, for example, on a common integrated circuit substrate. Forexample, many of the components of modulator section 603 may beimplemented by a digital signal processor and corresponding software.Similarly, if the modulator and feedback components of any of theforegoing amplifier systems are implemented in the analog domain, theymay be efficiently implemented on a common integrated circuit substrate.

FIG. 7 illustrates a number of interrelated operations that may be usedto implement one or more of the foregoing amplifiers. As shown, primaryinterleaved PWM pulses are generated from a modified input signal atblock 705. At block 710, the primary interleaved PWM pulses are used todrive the power stage of an interleaved PWM amplifier. Secondaryinterleaved PWM pulses are generated at block 715. The secondaryinterleaved pulses are generated using one or more carrier signals thatdiffer in phase from the carrier signal(s) used to generate the primaryinterleaved PWM pulses at block 705. At block 720, the modified inputsignal used at block 705 is generated from an input signal and both theprimary and the secondary interleaved PWM pulses.

FIG. 8 illustrates a further set of interrelated operations that may beused to implement one or more of the foregoing amplifiers. As shown,primary interleaved PWM pulses are generated from a modified inputsignal at block 805. At block 810, the primary interleaved PWM pulsesare used to drive the power stage of an interleaved PWM amplifier.Secondary interleaved PWM pulses are generated at block 815. Thesecondary interleaved pulses are generated using one or more carriersignals that differ in phase from the carrier signal(s) used to generatethe primary interleaved PWM pulses at block 805. At block 820, themodified input signal used at block 805 is generated by combining aninput signal, the secondary interleaved PWM pulses, and at feedbacksignal corresponding to the output signal from the power stage of theamplifier to, for example, a load.

Another amplifier system 900 that employs interleaved PWM signals forPWM ripple suppression is illustrated in FIG. 9. The system 900 may beimplemented in the analog domain, the digital domain, or combination ofboth. In this example, a signal 902 that is to be amplified by system900 is provided at the input of a converter 905. The output of theconverter 905 is provided to the input of an interleaved PWM generator907. When interleaved PWM generator 907 is implemented as a digitalcircuit, converter 905 may be in analog-to-digital converter having thefull resolution and linearity of the entire signal path through theamplifier system 900. In an analog implementation, converter 905 may beimplemented as a gain stage that adapts to the prevailing input levelsin filters the incoming signal to prevent aliasing with the PWMmodulation process.

The interleaved PWM generator 907 provides primary and secondary sets ofinterleaved PWM pulses in response to the signal provided from converter905. The primary set of interleaved PWM pulses are provided at lines 909and have an interleave order of N. The secondary set of interleaved PWMpulses are provided at lines 910 and have an interleave order of L. Theinterleaved PWM pulses of the primary set are generated by interleavedPWM generator 907 using carrier signals that differ in phase from thecarrier signals used to generate the interleaved PWM pulses of thesecondary set. In this example, interleave order N=L and N>1.

The primary set of interleaved PWM pulses are provided to the input ofan interleaved PWM corrector 912. The PWM corrector 912 adjusts thepulse widths of the primary set of interleaved PWM pulses in response toone or more correction signals provided at line(s) 914 to generatecorrected PWM drive signals N′. The corrected PWM drive signals N′, inturn, are used to drive the output switching transistors of aninterleaved PWM output stage 916. PWM modulated signals N″ are providedfrom the output of interleaved PWM output stage 916 through outputfilter 918 for supply to a load, such as a speaker.

The correction signal(s) at line(s) 914 is derived from one or morefeedforward signals corresponding to the secondary set of interleavedPWM pulses at lines 910. Additionally, the correction signal(s) may bederived from one or more feedback signals corresponding to the PWMmodulated signals N″ provided from the output of the interleaved PWMoutput stage 916. System 900 employs both feedforward and feedbackcircuit paths to facilitate derivation of the correction signal(s) fromboth the secondary set of interleaved PWM pulses and the PWM modulatedsignals N″. To this end, the secondary set of interleaved PWM pulses atlines 910 are provided to the input of converter 920. Converter 920provides a PWM waveform of appropriate response and amplitude to combinewith a scaled version of the PWM modulated signals N″. In a digitalimplementation, converter 920 converts a digital code to an analog pulsewaveform of related width. In an analog implementation, converter 920may perform signal scaling of the pulses at lines 910. The output ofconverter 920 may be provided to a positive terminal of a summingcircuit 922 through a gain stage 924 having a transfer function H_(L).The transfer function H_(L) may correspond to an attenuation and/orfiltering operation that is designed to ensure that a properrelationship exists between the secondary set of interleaved PWM pulsesand the PWM modulated signals N″. It will be recognized, however, thatany conversion, gain, and/or filtering operations may be executed in asingle functional block or divided in a different manner betweenmultiple functional blocks. Consequently, components used in thefeedforward path of system 900 merely illustrate one manner in whichsuch operations may be implemented.

The PWM modulated signals N″ may be provided to the input of a negativeterminal of summing circuit 922 through a gain stage 926. In thisexample, gain stage 926 has a transfer function H_(N). The transferfunction H_(N) may correspond to an attenuation and/or filteringoperation that is designed to ensure that a proper relationship existsbetween the PWM modulated signals N″ and the secondary set ofinterleaved PWM pulses.

The summing circuit 922 subtracts the signal at the output of the gainstage 926 from the output of the gain stage 924 to generate one or moreerror signals 928. The error signal(s) is a measure of the error betweenthe ideal PWM signal output provided from the interleaved PWM generator907 and the PWM modulated signals N″ provided at the output of theinterleaved PWM output stage 916. The error signal(s) 928 is amplifiedby an error amplifier 930 having a transfer function H_(E) andoptionally converted to an appropriate form by converter 932 forprovision to the interleaved PWM corrector 912. If the interleaved PWMcorrector 912 is implemented as a digital circuit, then converter 932may comprise an analog-to-digital converter that converts the analogoutput from the error amplifier 932 an appropriate digital format forinput to the interleaved PWM corrector 912. The conversion time of suchan analog-two-digital converter may be selected so that it is very fastso as to avoid adding large amounts of phase lag to the feedback loop.Additionally, the dynamic range of such a converter also may be large.While the magnitude of the error signal should be smaller than themagnitude of the main signal, the errors that result from unregulatedpower supplies are quite large. The transfer function H_(E) of the erroramplifier 930 cascaded with the gain of the interleaved PWM output stage916 and transfer function H_(N) should be selected to meet the Nyquiststability criteria.

The secondary set of interleaved PWM signals 910 may be delayed withrespect to the primary set of interleaved PWM signals 909 that aregenerated by the interleaved PWM generator 907. If N=L, then thesecondary set of interleaved PWM signals 910 may constitute a delayedversion of the primary set of interleaved PWM pulses. This delay may beintroduced, for example, by converter 920, gain stage 924, and/or usinga separate delay circuit. The magnitude of the delay may be selected tocorrespond to the expected signal delay through the interleaved PWMoutput stage 916. The primary set of interleaved PWM pulses may beexpected to be phased to evenly divide a unit circle. Thus, thesecondary set of interleaved PWM pulses, used as reference signals, alsomay be phased phase to evenly divide the unit circle, just slightlylagging in phase to the primary set of interleaved PWM pulses.

The error amplifier 930 may be implemented in a manner that facilitatescreation of a conditionally stable feedback system having a high orderof integration. To this end, the error amplifier 930 may be implementedusing a high order passive RC differentiator for the feedback around onehigh gain inverting amplifier. An active clamp may be used around thisfeedback network to suppress out-of-control outputs that occur duringsystem overload.

Gain stages 924 and 926 may combine their interleaved inputs internallyor pass them to their outputs for summing. Thus there are no signalcounts shown in FIG. 9 on the outputs for either stage. Both forms arelogically equivalent in a linear network. Both signal paths may have thesame relative amplitude and phase response to allow them to produce azero difference when the interleaved PWM output stage 916 is errorless.

The gain stage 926 also may receive low-pass filtered inputs 937 fromthe output filter 918. This may be done readily using high-order remotesensing methods where the high-frequency signal path comes from theinterleaved PWM output stage 916 and the low-frequency signal path comesfrom the output filter 918.

FIG. 10 is a flow chart showing a number of interelated operations thatmay be executed in a system employing interleaved PWM signals for PWMripple suppression. As shown, a first set of N interleaved PWM signalsare generated at block 1005 and a second set of L interleaved PWMsignals are generated at block 1010. The operations shown at blocks 1005and 1010 may be executed concurrently. The first set of interleaved PWMsignals at block 1005 are generated by modulating carrier signalsnumbering N with a signal that is to be amplified, and the second set ofinterleaved PWM signals at block 1010 are generated by modulatingcarrier signals numbering L with the signal that is to be amplified. Thecarrier signals used to generate the first set of interleaved PWMsignals may differ in phase from the carrier signals used to generatethe second set of interleaved PWM signals.

At block 1015, an error signal is generated. In this example, the errorsignal is derived from the output signals of the interleaved PWM outputstage and the second set of interleaved PWM signals that are generatedat block 1010. This error signal is used at block 1020 to correct thepulse widths of the first interleaved PWM signals. The correctedinterleaved PWM signals, in turn, are used to drive the interleaved PWMoutput stage of the amplifier at block 1025.

While various embodiments of the invention have been described, it willbe apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible within the scope of theinvention. For example, there are many embodiments of delay generatorsand how they may be modulated or bypassed. Accordingly, the invention isnot to be restricted except in light of the attached claims and theirequivalents.

1. An amplifier comprising: an interleaved PWM generator providingprimary and secondary sets of interleaved PWM pulses in response to aninput signal; an interleaved PWM output stage responsive to correctedinterleaved PWM pulses corresponding to the primary set of interleavedPWM pulses to generate an amplified PWM output signal; an interleavedPWM corrector coupled between the interleaved PWM generator and theinterleaved PWM output stage, the interleaved PWM corrector operable toreceive the primary set of interleaved PWM pulses, and the interleavedPWM corrector responsive to one or more correction signals derived fromone or more feedforward signals corresponding to the secondary set ofinterleaved PWM pulses to generate, from the received primaryinterleaved PWM pulses, the corrected interleaved PWM pulses for use bythe interleaved PWM output stage.
 2. An amplifier as claimed in claim 1,where the interleaved PWM pulses of the primary set are generated withcarrier signals that differ in phase from carrier signals used togenerate the interleaved PWM pulses of the secondary set.
 3. Anamplifier as claimed in claim 1 where the interleaved PWM corrector isfurther responsive to one or more feedback signals corresponding to theamplified PWM output signal to generate the corrected interleaved PWMpulses for use by the interleaved PWM output stage.
 4. An amplifier asclaimed in claim 1 where the primary set of interleaved PWM pulses hasan interleave order of N >1, the secondary set of interleaved PWM pulseshas an interleave order of L, and where N=L.
 5. An amplifier as claimedin claim 1 where the primary set of interleaved PWM pulses is of aninterleave order N>1, the secondary set of interleaved PWM pulses is ofan interleave order L, and where N does not equal L.
 6. An amplifiercomprising: an interleaved PWM generator providing primary and secondarysets of interleaved PWM pulses in response to an input signal; afeedforward circuit providing one or more feedforward signals derivedfrom the secondary set of interleaved PWM pulses; an interleaved PWMoutput stage responsive to corrected interleaved PWM pulsescorresponding to the primary set of interleaved PWM pulses to generatean amplified PWM output signal; a feedback circuit providing one or morefeedback signals derived from the amplified PWM output signal; aninterleaved PWM corrector coupled between the interleaved PWM generatorand the interleaved PWM output stage, the interleaved PWM correctoroperable to receive the primary set of interleaved PWM pulses, and theinterleaved PWM corrector responsive to one or more correction signalsderived from the one or more feedforward signals and the one or morefeedback signals to generate, from the received primary interleaved PWMpulses, the corrected interleaved PWM pulses for use by the interleavedPWM output stage.
 7. An amplifier as claimed in claim 6, where theinterleaved PWM pulses of the primary set are generated from carriersignals that differ in phase from carrier signals used to generate theinterleaved PWM pulses of the secondary set.
 8. An amplifier as claimedin claim 6 where the primary set of interleaved PWM pulses is of aninterleave order N, the secondary set of interleaved PWM pulses is of aninterleave order L, and where N=L.
 9. An amplifier as claimed in claim 6where the feedforward circuit comprises: a converter disposed to receivethe secondary set of interleaved PWM pulses; and a gain circuit having atransfer function H_(L).
 10. An amplifier as claimed in claim 9 wherethe feedback circuit comprises a gain circuit having a transfer functionH_(N).
 11. An amplifier as claimed in claim 10 and further comprising asumming circuit that provides one or more summed output signalscorresponding to a difference between one or more signals provided at anoutput of the gain circuit of the feedback circuit and one or moresignals provided at an output of the gain circuit of the feedforwardcircuit.
 12. An amplifier as claimed in claim 11 and further comprisinga signal processing circuit responsive to the summed output signal toprovide the one or more correction signals to the interleaved PWMcorrector.
 13. An amplifier comprising: an interleaved PWM generatorproviding a primary set of interleaved PWM pulses and a secondary set ofinterleaved PWM pulses in response to an input signal; an interleavedPWM corrector coupled with the interleaved PWM generator, theinterleaved PWM corrector operable to generate a corrected primary setof interleaved PWM pulses by adjustment of a width of pulses included inthe primary set of interleaved PWM pulses based on a correction signalderived from the secondary set of interleaved PWM pulses; and aninterleaved PWM output stage coupled with the interleaved PWM corrector,the interleaved PWM output stage operable to generate an amplified PWMoutput signal using the corrected primary set of interleaved PWM pulsesreceived from the interleaved PWM corrector.
 14. The amplifier of claim13, where the input signal is amplified by the amplifier and provided asthe amplified PWM output signal to drive a load.
 15. The amplifier ofclaim 13, where the interleaved PWM output stage comprises a pluralityof output switching transistors that are driven by the corrected primaryset of interleaved PWM pulses to generate the amplified PWM outputsignal used to drive a load.
 16. The amplifier of claim 15, where thesecond set of interleaved PWM pulses are used to derive feedforwardsignals, the PWM modulated signals are used to derive feedback signals,and the correction signal is derived from the feedforward signals andthe feedback signals.
 17. The amplifier of claim 16, where thefeedforward signal are subtracted from the feedback signals to derivethe correction signal.
 18. The amplifier of claim 13, where the primaryset of interleaved PWM pulses and the secondary set of interleaved PWMpulses are generated concurrently by the interleaved PWM generator. 19.The amplifier of claim 13, where the primary set of interleaved PWMpulses are generated with the interleaved PWM generator by modulation ofa first carrier signal comprising N carrier signals with the inputsignal.
 20. The amplifier of claim 19, where the secondary set ofinterleaved PWM pulses are generated with the interleaved PWM generatorby modulation of a second carrier signal comprising L carrier signalswith the input signal, the first carrier signal differing in phase fromthe second carrier signal.